A pressure switch is a device that closes or opens an electrical contact when a pressure is above or below a certain preset pressure threshold. Pressure switches can be used in conjunction with pressure sensors, wherein the pressure sensor provides an output voltage proportional to the magnitude of an applied pressure. Pressure switches are used in a variety of different settings including manufacturing plants, automobiles, aircraft, and heavy machinery.
A sample application for a pressure switch, particularly a solid state pressure switch, is as part of a monitoring system for an oil filter. In such an application, the switch is configured to engage a circuit that powers an indicator light, or other warning system, when the differential pressure across the oil filter exceeds a predetermined threshold. Exceeding the predetermined threshold is usually an indication that the oil filter is clogged and requires replacement.
Conventional pressure switches, however, perform inadequately at low temperatures. As the temperature of oil drops it becomes more viscous. The pressure across the oil filter will be higher due to the higher viscosity of the oil. This condition can lead to false positives, where the pressure switch engages the indicator light because of high pressures, even though the oil filter is not actually clogged. As a result, perfectly usable oil filters are discarded and replaced.
FIG. 1 illustrates a conventional pressure switch. A pressure sensor 110 outputs a voltage indicating the amount of pressure being measured, and that output is amplified 120. A comparator 130 compares the output of the pressure sensor against a pre-set reference voltage provided by a threshold setting network 140. The reference voltage represents a voltage level that, when exceeded by the output of the pressure sensor 110, implies that some corrective action needs to be taken. Since the conventional pressure switch makes no accounting for temperature, a drop in the temperature of the liquid being measured will lead to higher viscosity of the liquid and can cause the pressure measured by the pressure sensor 110 to exceed the reference voltage. If this scenario occurs, a warning will be triggered even though no corrective actions are necessary.
Accordingly, there is a need for a pressure switch that takes into account the temperature and corresponding viscosity of the liquid medium being measured.